71b 4K->32K Ram module conversion



#4

After hearing about increasing the memory in a 42s, I started thinking why not try it with a 71b 4K ram module.

Do any of the experts out there know if this is within reason? I've never opened up a 4K ram module but I have 4 of them. If I could swap out the 4K chip for a 32...or 64K chip I would be delighted!

Andy


#5

I've been reading the 71B hardware internal design specs on the new Museum CD's and I'll bet that the 4KB RAM module contains one of the four 4KB hybrids that are built in to the 71B. Each of these hybrids is in turn made of four 1KB RAM chips that are designed specifically for the 71B bus. Again, based on my understanding of the spec - not certain knowledge - the 1KB RAM chips are soft configured devices that are daisychained together on the hybrids and the 4KB hybrids are designed so that they can in turn be daisychained, as they are inside the 71B, to make 16KB of RAM which, along with 1.5KB from the display driver chips, adds up to the 17.5KB standard RAM of the 71B. So I think you could daisychain multiple 4KB RAM modules and plug the assembly into one 71B port - not just 4 modules, there should be no logical limit but the CPU's addressing range, although the capacitive loading of paralleled hybrids would set a pretty strict limit. But I don't think you would find a parallel bus with separate address and data and chip select signals to connect a standard memory chip to - in the HP 4K module. The third party memory modules from 32K on up probably do have standard memory chips inside them (again, I'm guessing) and it might be possible to upgrade them, if their designs provide the additional address lines or chip selects required.


#6

I beleive you're correct about the HP modules. Some 3rd party modules contain that HP Saturn Bus Interface chip (the 1LJ4 or whatever it's called). AFAIK, when addressing RAM, this chip can connect to 4 standard 8K SRAM chips, but nothing more (neither more chips nor larger chips).
Which means these modules are pretty much limited to 32K.
I assuem the front port 32K modules use some custom interface chip (there's not enough space in there for the 1LJ4 and 4 RAM chips), but again I think upgrading them is going to be impossible.


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